1. Field of the Invention
The present invention relates generally to optical printing heads comprising a plurality of light emitting diodes (LED), and more particularly, to the improvement of the so-called dynamic driving type optical printing head.
2. Description of the Related Art
An example of a printing system including an optical printing head having a LED array is described in U.S. Pat. No. 3,850,517, for example.
In general, an optical printing head having a LED array comprises a plurality of LED blocks arranged along the main scanning direction of the optical printer. Each of the LED blocks comprises a plurality of LED elements having an one-to-one correspondence with the printing dots of the main scanning direction. It is therefore possible to control the selected desired LED elements to be energized simultaneously in the main scanning direction. For the purpose of realizing this advantage, a static driving type optical printing head comprising a driver having the number of bits identical to the number of the LED elements is used in practice.
An optical printing head utilizing LED array also has other advantages. These advantages include the precise control of the printing dot shape and dot location, as well as the dispensability of mechanical scanning means in the main scanning direction. Consequently, not only a printing optical head of the static driving type, but also an optical printing head of the dynamic driving type energizing LED elements in time-division has been developed. An example of a dynamic driving type optical printing head is disclosed in Japanese Patent Laying-Open No. 63-254068, for example.
FIG. 1A is a schematic top view showing the dynamic driving type optical printing head disclosed in Japanese Patent Laying-Open No. 63-254068, whereas FIG. 1B is a sectional view taken along line 1B--1B of FIG. 1A. A dynamic driving type optical printing head requires many conductor lines to be formed in high density on a head substrate 1. Particularly, selecting signal lines 41, 43 are formed in a matrix manner for selectively energizing the desired elements of the many LED elements 22.
In general, LED elements 22 are grouped by LED blocks 2 and dynamically driven in time-division. A common electrode 13 is formed on head substrate 1 for each LED block 2, whereby the bottom of each LED block 2 is joined to the corresponding common electrode 13 with conductive adhesive.
Within one LED block 2, the selected desired LED elements 22 are energized simultaneously. In the case where one LED block 2 comprises 64 LED elements 22 for example, 64 selecting signal lines are necessary to connect the LED elements 22 individually to a data driver 3. In FIG. 1A, the selecting signal lines comprise 64 bridging lines 41 provided in parallel with the array of the LED elements, and selecting lead lines 43 connecting each of the 64 LED elements 22 of LED block 2 to the corresponding bridging line 41.
Selecting lead lines 43 provided in groups of 64 lines per period for every LED block 2 are formed directly on head substrate 1 and arranged in parallel with each other in a pitch equal to the pitch of the LED element 22 array. Each of selecting lead lines 43 is covered by an interlayer insulator film 42, except for the portion in the proximity of LED element 22. Bridging lines 41 are formed on interlayer insulator film 42, with one end of each selecting lead line 43 connected to the corresponding bridging line 41 via a contact hole 42H. The other end of each selecting lead line 43 is connected to the corresponding LED element electrode 23 by wire bonding. One end of each bridging line 41 is connected to data driver 3 by wire bonding. Accordingly, LED element 22 of each LED block 2 is individually connected to data driver 3 via the corresponding selecting lead line 43 and bridging line 41.
The printing head in accordance with the above described prior art may use a glass plate or a sintered ceramic plate for head substrate 1. In the case a glass head substrate 1 is used, selecting signal lines 41, 43 may be formed under thin film technology by sputtering or evaporation because the surface of the substrate is smooth. It is therefore possible to form bridging lines 41 in density to reduce the width of head substrate 1. However, a large vacuum apparatus is required for the purpose of sputtering or evaporation because the entire head substrate 1 has a large dimension. A large and precise apparatus is also required for precise patterning of the sputtered or evaporated thin film. There is also a disadvantage that the glass head substrate 1 is liable to cracking due to its long length, which will be aggravated if the width is further decreased.
In the case where head substrate 1 of sintered ceramic is used, the strength of head substrate 1 is high, but the smoothness of the substrate surface is low. Therefore, selecting signal lines 41, 43 are formed under thick film technology using silk-screen printing method. Consequently, the width of the bundle of bridging lines 41 will increase to expand the entire width of the printing head. It is preferred that the width of the printing head is small because not only the printing head, but also many other devices such as a charger, a toner supply device, a transfer device, a cleaner, etc. are arranged in the periphery of the optical printer's photoreceptor drum.
There is also a problem that the bundle of bridging lines 41 formed of very long fine lines (generally exceeding at least 200 mm) parallel to each other in high density is susceptible to shorts and disconnections. When such shorts and disconnections occur, partial repair is difficult, leading to the discard of the entire head substrate 1.
Furthermore, when the number of the LED blocks used is changed, a head substrate 1 having a bundle of bridging lines 41 formed with the corresponding change in length must be provided.
In addition, bonding operation by wiring means, such as wire bonding or TAB (tape-automated-bonding) between selecting lead line 43 and LED element electrode 23 is not easy due to the difference in their heights, as can be seen from FIG. 1B.